Broadly described, the invention relates to a well known type of safety shut-off valve wherein the flow of pressurized fluid is controlled by a gate on a valve stem that extends through the bonnet of the valve to a piston in an outer control chamber. The shut-off valve is opened by the pressure of control fluid on the piston and in the absence of such control pressure the valve is closed in a fail-safe manner by line pressure on the valve and/or spring pressure on the piston.
The present invention is directed to certain needs for improvement in the construction and operation of such a safety shut-off valve in the petroleum and related industries.